From the site: “A Digital Raster Graphic (DRG) is a scanned image of a U.S. Geological Survey (USGS) topographic map. An unclipped scanned image includes all marginal information, while a clipped or seamless scanned image clips off the collar information. DRGs may be used as a source or background layer in a geographic information system, as a means to perform quality assurance on other digital products, and as a source for the collection and revision of digital line graph data. The DRGs also can be merged with other digital data (e.g., digital elevation model or digital orthophotoquad data), to produce a hybrid digital file.

The output resolution of a DRG varies from 250 to 500 dots per inch. The horizontal positional accuracy of the DRG matches the accuracy of the published source map. To be consistent with other USGS digital data, the image is cast on the UTM projection, and therefore, will not always be consistent with the credit note on the image collar. Only the area inside the map neatline is georeferenced, so minor distortion of the text may occur in the map collar. Refer to the scanned map collar or online Map List for the currentness of the DRG.”